This is a request for Phase I funding to test the hypothesis that glycoprotein (GP) V is the thrombin receptor on platelets. Recent data indicate that thrombin, acting proteolytically at a receptor on the platelet surface, is the physiologically important agonist to activate platelets and to initiate arterial thrombus formation. Previous studies from the applicant's laboratory have shown that the only detectable thrombin substrate on the platelet membrane surface is GP V. In the proposed study, GP V will be cloned to deduce the amino acid sequence of this protein.